A resistance-change memory (resistive random access memory [ReRAM]) has been proposed as a memory element that uses the resistance switching phenomenon of a variable resistance film formed between upper and lower electrodes in information recording. An example of the ReRAM is a filament-operation-type memory using a transition metal oxide as the variable resistance film. In this filament-operation-type ReRAM, a thin conduction path (filament) is formed in order to perform switching with a low current. To form the filament, however, it is necessary to divisionally apply a voltage little by little a plurality of number of times in initialization (forming) of the element. Therefore, the forming before the shipment of the product takes a long time, and this interferes with the shipment of the product. To prevent this, attempts have been made to shorten the forming time by using a variable resistance film that facilitates forming.
“IEDM Tech. Dig., p. 297, 2008” describes the technique of lowering the forming voltage by thinning the variable resistance film. More specifically, a thin transition metal oxide deposited by chemical vapor deposition (CVD) method and having a thickness of about 5 nm or less is used as the variable resistance film. In this method, however, as the film thickness decreases, it becomes difficult to form a metal-rich film (a film in which the composition ratio [O]/[Me] of oxygen to a metal is lower than the stoichiometric ratio) necessary for a stable switching operation. That is, “IEDM Tech. Dig., p. 297, 2008” can lower the forming voltage, but cannot obtain stable switching characteristics (switching yield and durability [endurance characteristics]).
Also, “Symposium on VLSI Technology, p. 30, 2009” describes the method of forming a variable resistance film by metal sputtering and thermal oxidation following the sputtering. More specifically, a metal film is formed by metal sputtering and oxidized by thermal oxidation. A thin transition metal oxide thus formed and having a thickness of about 5 nm or less is used as the variable resistance film. When forming the variable resistance film (metal oxide film) by thermal oxidation, however, the lower portion (near the interface with a lower electrode) of the variable resistance film is readily oxidized. This degrades the adhesion between the variable resistance film and lower electrode. That is, in “Symposium on VLSI Technology, p. 30, 2009”, a metal-rich film is obtained by controlling thermal oxidation, but film peeling readily occurs in the interface between the variable resistance film and lower electrode. In addition, in the method using thermal oxidation, the oxygen amount in the direction of film thickness is difficult to control. Therefore, a film is relatively uniformly oxidized toward the depth of the film. This makes a low-current switching operation difficult.